Spectrophotometry concerns the study of the optical properties of a sample at various wavelengths. Typically, measurements are made of variations in the intensity of light either reflected from or transmitted through a sample as a function of wavelength. The resulting data can be used to determine the composition of the sample and the relative concentrations of the constituents making up the sample. This analysis is usually accomplished by comparing a spectral pattern of light intensities as a function of wavelength for the sample with the spectral patterns of known substances. The comparison process itself often involves complex mathematical techniques such as the use of Partial Least Squares With Latent Variables to match the spectral pattern of the known reference substances with the pattern of the sample.
Spectrophotometry has a wide variety of applications in both science and industry. In the latter usage, spectrophotometry has been used to determine the moisture and protein content of various grains and monitor product composition in flowing chemical production processes.
Spectrophotometers typically include a light source, a grating for dispersing light in a series of monochromatic, single wavelength, beams and some type of photodetector. The grating may be positioned to provide pre-dispersed monochromatic light to both the sample and the detector or, alternatively, polychromatic light from the source may be directed onto the sample and then dispersed by the grating before being directed to the detector. Since gratings commonly disperse light of varying wavelength at differing angles, the wavelength of the monochromatic light beam which eventually reaches the detector in either spectrophotometer configuration may be changed by simply rotating the grating. Fiber optic cables can also be employed to conduct light from the spectrophotometer to a distant sample. This approach has considerable advantages in industrial chemical processing applications.
Since spectrophotometric analysis depends upon a comparison of relative light intensities, it is desirable to maintain the intensity of light reaching the sample as uniformly constant as possible, both over the range of wavelengths being employed by the spectrophotometer and over the passage of time. The accuracy of a spectrophotometer is often characterized by the minimum intensity of light from the apparatus which fluctuates in a manner that cannot be otherwise compensated for.
One approach to enhancing the accuracy of a spectrophotometer is to maximize the uniformity of light produced within and conducted through the apparatus. Previous efforts have been made, for example, to develop a polychromatic light source having a highly uniform output. One example of this approach is illustrated in U.S. Pat. No. 4,094,609 to Y. Fujii, et al. Unfortunately, however, even the best light sources have discernible variations in light intensity at differing wavelengths which also change over time.
In addition to minimizing known sources of light intensity drift within a spectrophotometer, efforts have also been made to compensate for this drift by providing a reference spectral pattern which can be used analytically to account for variations in the light intensity of the sample spectral pattern which are not attributable to light interaction with the sample. In one application of this approach, illustrated in U.S. Pat. No. 4,696,570 to Joliot, et al., pre-dispersed monochromatic light from a grating is directed onto a fiber optic bundle which conducts the light to a sample. A small portion of the fiber optic bundle is also used to bypass the sample and conduct a fraction of the pre-dispersed monochromatic light onto a reference sample and then a detector. The spectral pattern from the reference sample is then used to compensate for variations in the intensity of the pre-dispersed monochromatic light beam entering the fiber optic bundle. This approach, however, still suffers from some inaccuracy.
In another application of the reference beam approach, illustrated in U.S. Pat. Nos. 4,285,596 and 4,540,282 to Lamda, et al., the sample under study is periodically replaced with a sample of known optical properties, such as teflon, to compensate for long term variations in light intensity as a function of wavelength. This approach requires a high degree of conformity in determining the wavelength at which a particular intensity measurement is made. The two Lamda, et al., patents are thus directed to grating drive systems suitable for repetitive determination of grating orientation in order to repetitively determine the wavelengths at which various light intensity measurements are made. In the former patent, a pair of complex configured cam members are employed, while in the latter patent, a two-pole brushless DC motor and flat return spring are used.
Periodically replacing a sample under study with a known reference material, however, can be inconvenient in a number of conventional spectrophotometric applications. In the case of flowing chemical production processes, for example, the flow of fluid under study must be blocked in order to provide an air reference sample. Thus the replacement process is typically time consuming and may also be labor intensive. Additionally, because of the speed of the grating drive system which is necessary to achieve a suitable repetitive accuracy, the duration and hence intensity of the light at any particular wavelength interacting with the sample may be undesirably low.
Thus there still exists a need for a spectrophotometric measurement apparatus and methodology which can provide enhanced accuracy. The present invention fulfills this need.